Fix compiler warning and potential heap corruption due to mismatched new/delete

[meyerj]

This fixes a -Wmismatched-new-delete compiler warning with Apple LLVM version 9.0.0 (clang-900.0.39.2) in macOS Sierra:

[ 77%] Building CXX object src/rviz/default_plugin/CMakeFiles/rviz_default_plugin.dir/map_display.cpp.o
/Users/vagrant/ros_kinetic_desktop_ws/src/rviz/src/rviz/default_plugin/map_display.cpp:213:3: warning: 'delete' applied to a pointer that was allocated with 'new[]'; did you mean 'delete[]'? [-Wmismatched-new-delete]
  delete pixels;
  ^
        []
/Users/vagrant/ros_kinetic_desktop_ws/src/rviz/src/rviz/default_plugin/map_display.cpp:183:27: note: allocated with 'new[]' here
  unsigned char* pixels = new unsigned char[pixels_size];
                          ^

According to the C++ FAQ and the critical C++ FQA this is undefined behavior and can potentially lead to heap corruption. Probably it would be a good idea to use boost::shared_array instead for exception safety.

Note that I did not look into the source code in detail or check whether and when the Swatch::updateData() function is called.

[wjwwood]

lgtm, thanks! I'll just wait for CI to catch up before merging.